This invention relates to a thermal bridge system and more particularly to such a thermal bridge system which can selectively either thermally isolate or thermally connect a warm object and a cool object without any immediate/short term or long-term degradation in thermal conductivity between the objects.
Thermoelectric chips (xe2x80x9cTECsxe2x80x9d) chips are utilized in various cooling and heating applications. These TECs are actually miniature solid state heating/cooling devices which have no moving parts yet perform the function of drastically cooling one side of the chip while producing a proportionate increase in temperature on the other side of the chip. TECs function through what is known as the Peltier effect when current passes through the junction of two different types of conductors it results in a temperature change. Today, Bismuth Telluride is primarily used as the semiconductor material, heavily doped to create either an excess (N-type) or a deficiency (P-type) of electrons. Essentially, when a DC current passes through the junction of two wires made of dissimilar metals, the wire portions made of the first metal tend to heat up while the wire portions of the second metal tend to cool down. Correspondingly, if the current (polarity) is reversed, the heat is moved in the opposite direction. In other words, what was the hot face will become the cold face and vice a versa.
Very simply, a TEC consists of a number of P- and A-type pairs (couples) connected electrically in series and sandwiched between two ceramic plates. The cooling wire portions are all attached to a first ceramic plate (the cooling plate) and the warming wire portions are all attached to a second ceramic plate (the warming plate), where an air gap is kept between these two plates to act as an insulator. Precautionary measures are taken to insure that no water or condensation forms in between these two ceramic plates, as the water would act as a conductor and would short the heating/cooling wire portions.
When designed into systems, the warm ceramic plate of the TE chip is attached to a heat sink while the cool ceramic plate of the TE chip is attached to a device known as a cooling shoe, which absorbs latent heat from a medium. Typically, the cooling shoe is designed in a shape to accept or receive the shape of the object being cooled. For example, if the cooling shoe is designed to cool a can of soda, the cooling shoe would typically have a semicircular, concave shape so that the can of soda would fit into the cavity of the cooling shoe. This design feature is to effectively maximize surface contact, i.e. assist in cold transfer. Typical embodiments for these TE chip/heat sink/cooling shoe systems would be small-volume cooling systems, such as cooler chests or soda machines.
Thermodynamic principles mandate that the heat sink be spaced in optimal distance apart from the cooling shoe to prevent any convective heating of the cooling shoe. This optimal distance is typically two inches. Therefore, a spacer known as a bridge is typically placed between the cool ceramic plate of the TE chip and the cooling shoe. Further, rigid insulation or any other insulative material is utilized to insulate the bridge/TE chip structure so that convective heat transfer between the heat sink and the cooling shoe is minimized.
Please note that TE chips only function when a DC current is pumped through the heating/cooling wire portions within the chip. In the event of a power failure (or any other occurrence which interrupts current flow through the chip), the TE chip ceases to function as a heating/cooling device and, through conduction between the two ceramic plates via the heating/cooling wire portions, attempts to equalize the ceramic plate temperatures. Therefore, when no power is applied to the TE chip, the cooling shoe will warm up and the heat sink will cool down until they are at equal temperatures. Naturally, this is highly undesirable, as typical applications for TE chip-based cooling systems must maintain a specific temperature inside of the space being cooled. This situation is only aggravated by the fact that the power provided to these TE chips is typically cycled so that the temperature inside of the area being cooled is maintained within a predetermined range. In the event that the temperature within the area being cooled drops below the lower temperature of that predetermined range, power would then be cut to the TE chip. Unfortunately, this would result in the TE chip no longer functioning as a cooling device and actually (through conduction) equalizing the temperature of its plates and, therefore, the heat sink and cooling shoe. Accordingly, the temperature inside the cool space would immediately start to rise until that temperature exceeds the high temperature of the predetermined range. At that point in time, power to the TE chip would be cycled on and the cool space would immediately start to be cooled down. This system would continuously cycle, where the TE chip is either cooling tile space (through active cooling) or heating the space (through conductive heat transfer).
In an attempt to minimize or eliminate this undesirable situation, separation of the TE chip from either the heat sink or the bridge has been experimental and unfortunately there are several problems associated with this practice. When working with TE chips, it is imperative that a thermally efficient connection be made between the TE chip and any surface to which it is attached. Typically, a dielectric grease is utilized to connect the chip to the heat sink and the bridge. Unfortunately, by physically separating the TE chip from either the bridge or the heat sink, due to the viscous characteristics of the dielectric grease, the grease tends to stretch out in a string fashion to bridge the gap introduced between the TE chip and the body to which it is attached. Naturally, this results in a system in which the chip is not fully insulated (or isolated) from the object to which it is attached if the distance is limited. Therefore, the intended purpose of this gap (namely to thermally isolate the TE chip from either the bridge or the heat sink to prevent the equalizing of the temperatures of the cooling shoe and the heat sink) is frustrated as the thermal energy will merely transfer through these finger-like grease extrusions. Therefore, the temperature of the cooling shoe and heat sink will equalize.
Additionally, when the TE chip is placed back into position against either the bridge or the heat sink, the compression of the finger-like grease extrusions will result in the introduction of air pockets into the grease itself. These air pockets (or bubbles) act like little insulating bodies embedded within the grease, lowering the thermal efficiency of the conductive path of the heating/cooling device itself.
Another attempt to minimize the introduction of heat into the cooled area involved the use of an insulating cover placed over the heat sink, the cooling shoe, or both. If this insulating cover is placed over the heat sink, the only heat introduced into the cool area would be the latent heat stored in the heat sink itself. Alternatively, if this insulating cover is placed over the cooling shoe, limited heat gain would be introduced into the cool area. However, neither one of these situations really solves the problem at hand, as it is usually impossible to get to either the cooling shoe or heat shoe to install an insulating cover. Additionally, concerning covering either the heat sink or cold shoe with an insulating cover, this would tend to be a highly mechanical and complicated process and the net result would be insufficient.
The present invention provides a thermal bridge system comprising a first thermally conductive surface positioned proximate an object which absorbs energy and a second thermally conductive surface in thermal communication with the first conductive surface. The second surface is positioned proximate an object which dissipates energy. The thermal bridge is also equipped with a thermal switch comprising a conductive path in communication with the first thermally conductive surface and the second surface by alternatively switching between a first position, blocking at least part of the conductive path and thermally insulating the first conduction surface from the second conductive surface and a second position, opening at least part of the conductive path and thermally connecting at least part of the first conductive surface with the second conductive surface.
The advantage offered by this thermal bridge design can be shown clearly in the use of medical transportation chest. These containers are carried in automobiles, vans, planes or other vehicles. The TEC utilizes DC current provided by the vehicles battery which is continuously recharged while the vehicle is in operation. Sometimes in the course of pickup delivery it may be necessary for the vehicle to be turned off. Consequently, the TEC would contribute to an undesirable drain on the battery thereby jeopardizing the ability to restart or use the vehicle. It therefore is prudent to discontinue providing power/DC current to the TEC. The existing science of TEC Applications then create a failure of the medical box to provide a secure cold environment. The design of the present invention would provide an improved mechanism to allow the medical box to retain the coldness preferred for sensitive samples.
In one embodiment of the present invention, the switch may be an insulating cylinder having a conductive passage. The switch may also be a conductive cylinder having an insulating material covering a radial portion of the cylinder""s surface. The switch may be a disk having a conductive angular portion and a non-conductive angular portion. The switch may be a sliding planar surface having a conductive portion and a non-conductive portion. The switch may be an insulting sphere having a conductive passage.
The switch may include a first switch surface positioned proximate the first conductive surface. The gapless thermal switch may also include a second switch surface positioned proximate the second conductive surface.
The interruptible thermal bridge system may include a conductive fluid positioned between the first conductive surface and the first switch surface. The conductive fluid may also be positioned between the second conductive surface and the second switch surface. The conductive fluid may be dielectric grease, a glycol-based fluid or a carbon-based fluid or any other highly conductive fluid. The interruptible thermal bridge system may include an actuator for selectively activating and deactivating the gapless thermal switch.
The interruptible thermal bridge system may include a cooling thermostat for deactivating the gapless thermal switch when the temperature proximate the cool object is above a cooling hi-point temperature which is the lowest temperature desired, thus allowing the energy absorbed by the cool object to be dissipated by the warm object. The cooling thermostat may deactivate the gapless thermal switch when the temperature proximate the cool object is below a cooling low-point temperature, thus allowing the energy absorbed by the cool object to be dissipated by the warm object. The interruptible thermal bridge system may include a heating thermostat for deactivating the gapless thermal switch when the temperature proximate the warm object is below a heating low-point temperature which is the warmest temperature in a gas range, thus allowing the energy absorbed by the cool object to be dissipated by the warm object. The heating thermostat may activate the thermal switch when tile temperature proximate the warm object is above a heating hi-point temperature, thus preventing the energy absorbed by the cool object from being dissipated by the warm object.
The present invention also provides a thermoelectric temperature control system comprising a cooling shoe positioned proximate a cool medium for absorbing energy from the cool medium and a heat sink positioned proximate a warm medium. The thermoelectric temperature control system also comprises a thermoelectric cooling device in thermal contact with and positioned proximate to the heat sink and an interruptible thermal bridge system position between the cooling shoe and the thermoelectric cooling device. The interruptible thermal bridge selectively insulates the cooling shoe from the thermoelectric cooling device.
In a preferred embodiment, the interruptible thermal bridge system may include: a first thermally conductive object having a first conductive surface, positioned proximate the cooling shoe; a second thermally conductive object having a second conductive surface thermally connected to the first conductive surface, positioned proximate the thermoelectric cooling device; and a gapless thermal switch positioned between the first and second conductive surfaces for selectively insulting the first conductive surface from the second conductive surface while maintaining a gapless connection between the conductive surfaces and the gapless terminal switch, thus selectively insulating the cooling shoe from the thermoelectric cooling device. The cool medium may be air and the system may include a first fan positioned proximate the cooling shoe for moving the cool medium over the cooling shoe to aid in the cooling shoe absorbing energy from the cool medium. The warm medium may be air and the system may include a second fan positioned proximate the heat sink for moving the warm medium over the heat sink to aid in the heat sink dissipating energy to the warm medium making the medium even warmer. The gapless thermal switch may include a first switch surface positioned proximate the first conductive surface.
The gapless thermal switch may include a second switch surface positioned proximate the second conductive surface. A conductive fluid may be positioned between the first conductive surface and the first switch surface. The conductive fluid may also be positioned between the second conductive surface and the second switch surface. The conductive fluid may be a dielectric grease, a glycol-based fluid, or a carbon-based fluid.
The thermoelectric temperature control system may include an actuator for selectively activating and deactivating the gapless thermal switch. The thermoelectric temperature control system may include a cooling thermostat for energizing the thermoelectric cooling chip and deactivating the gapless thermal switch when the temperature of the cool medium is above a cooling hi-point temperature, thus allowing the energy absorbed by the cooling shoe to be dissipated by the heat sink. The cooling thermostat may de-energize the thermoelectric cooling chip and activate the gapless thermal switch when the temperature of the cool medium is below a cooling low-point temperature, thus preventing the energy absorbed by the cooling shoe from being dissipated by the heat sink.
The thermoelectric temperature control system may include a heating thermostat for energizing the thermoelectric heating chip and deactivating the gapless thermal switch when the temperature of the warm medium is below a predetermined heating low-point temperature, whereby preventing the energy generated by the hot shoe to be dissipated by the cooling sink. The heating thermostat may de-energize the thermoelectric heating chip and activate the gapless thermal switch when the temperature of the warm medium is above a predetermined heating hipoint temperature, whereby allowing the energy absorbed by the heating shoe to be dissipated by the cold sink. Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: